how comfortable are you with a little hacking...?
if you want to try to get more informaiton go to xterm
sudo gainroot (or however you prefer to get root access)
vi /opt/witter/witter.py
scroll down to line 219 which should be the 'enterPressed' method

go into insert mode (i)
and add some print lines either side of the call to newTweet, like so:
print "sending tweet"

result = self.activeAccount.newTweet(self.ui.getEntryText() , reply_to_name=self.reply_to_name, reply_to_id=self.reply_to)

print "Tweet Sent"

then press ESC, and then :wq
to save and exit vi
Then re-run witter. This should at least show if the event notification is triggering the method at all.
also in that method immediately after that there is an if (result=True)
statement, if you put in there
print "tweet successful"
and in the else block
print "tweet failed"
then if for some reason the banner stuff isn't working, then the output should at least show whether witter things it's succeeding in posting or not.

If you don't fancy doing that, then you may have to wait until I get around to a 0.3.1-6 at which point I have put those changes in the code.